Cathodic protection has been used since 1824, when it was introduced by Sir Humphrey Davy as a way to protect copper sheathing in ships. More recently, cathodic protection has been used by the U.S. Army Corps of Engineers on lock gates on the Mississippi River to extend the effective life of paint coatings and to provide supplemental protection to defects in the paint film on immersed steel surfaces. The paint film, which is a hydraulic structure's primary protection against corrosion, is never perfect as defects and holidays are always present, and hard-to-paint areas such as edges, rivet heads, and weld beads may never receive adequate paint protection. In addition, paint film is scratched by debris and barge traffic on the waterways; the exposed steel then corrodes until repainted or repaired.
Most hydraulic structures are dewatered only every 10 to 15 years to inspect seals and pintels, and for general inspection and any needed repainting. Cathodic protection can provide supplemental protection to inadequately painted and/or scratched areas between dewatering cycles. Cathodic protection provides supplemental protection to paint coatings and thereby extends the life of the coatings. Furthermore, it decreases the time the structure must be out of service during interim inspections and can increase the total life of the structure.
The two basic types of cathodic protection anodes are sacrificial anodes and impressed current anodes. The sacrificial anode, which is attached to the metal structure to be protected, slowly dissolves to release electrons which flow to the metal structure to maintain the metal structure as a cathode, thereby protecting the metal structure. The impressed current anode preferably has a very low dissolution rate as the protection of the metal structure is achieved through the application of an impressed current from an external source to the anode.
Often, due to shortcomings in the anode materials and/or the configurations used, sacrificial and impressed current cathodic protection systems fail to operate effectively in mitigating corrosion. Sacrificial anodes normally have high material dissolution rates and therefore can corrode away before routine maintenance of the cathodic protection system can take place (i.e., during the dewatering cycle of a hydraulic structure). The "throwing power" of the sacrificial anodes is often insufficient to protect reasonable sized areas of the structure when the resistance of the water is greater than 3000 ohms-cm. In response to these problems, a large number of heavy bulky anodes must often be used for protection, putting a great deal of strain on structural supports and operating equipment. The large anodes are prone to installation problems because of their size and weight. They are also vulnerable to mechanical damage from floating debris or ice.
Anodes used in impressed current cathodic protection systems also have fundamental problems. Graphite anodes, over a period of time, undergo "degraphitization", where the binder material present in the anode is leached out, leaving behind a porous material that occupies the same column as the original anode. As the anode become less dense, the electrical connections fail, even in less severe environments. Silicon-iron anodes also have difficulties with their electrical connections. Anode connections made during installation do not provide adequate electrical isolation from the cathodically protected structure, and therefore electrical shorting takes place. In addition, graphite and silicon-iron anodes are still necessarily heavy (anodes weighing 18 lbs or more are common) due to their relatively high dissolution rates; this makes anode installation extremely difficult in most situations. Because of their size, graphite and silicon-iron anodes are also quite susceptible to mechanical damage in situations where ice and/or floating debris is present.
Platinized anodes consisting of a thin layer of a platinum group metal or oxide on a valve metal substrate such as titanium, niobium, hafnium, tantalum and alloys thereof are also available. The platinum group metals include platinum, ruthenium, osmium, and iridium. However, they are expensive and the thin coating is susceptible to erosion or abrasion damage in high velocity water. Furthermore, dissolution is accelerated by the AC ripple effect imposed by the rectifier.
U.S. Pat. No. 4,187,164 discloses a button of titanium or niobium partially imbedded in plastic material, with a layer of platinum on the exposed face of the button. The patentee noted that such construction is difficult to manufacture, and further that long, thin cathodic protection anodes are more effective than short, fat ones. The patentee indicated that long, thin anodes are difficult to mount and that it had been the practice to produce anodes assemblies which can be mounted in plastic material, such as the undulating wire anode, the rectangular bar having bat-wing plates attached along its length, and the rod or tubular anode. The patentee proposed a roll-formed titanium strip having a central U-shaped spine and longitudinally extending flanges, with the platinum or other active anode component being coated only along the convex spine. However, this latter design requires a substantial amount of expensive titanium to provide the uncoated portion of the spine and the flanges in comparison to the small extent of the platinized portion of the spine.
Conducting ceramic coatings are a relatively new concept in the field of impressed current anodes. The conducting ceramic anode coating must provide an effective barrier to oxygen ions, so that the substrate metal does not become oxidized. In addition, the ceramic coating must have a relatively high electron conductivity on active surface area for oxidation to occur, be mechanically strong and have good adherence to the substrate. U.S. Pat. No. 3,850,701 describes the use of a magnetite coating which was chemically processed over a titanium or tantalum substrate. However, the resulting coating had insufficient adhesion to the substrate.
U.S. Pat. No. 4,445,989 discloses the use of a second type of ceramic anode. It employs a cathodic anode comprising an electrically conducting ceramic coating on a valve metal substrate, where the consumption rate of the ceramic coating is on the order of 1 gram per ampere-year. The ceramic coating is approximately 10 to 20 mils thick and is produced by plasma spraying. The ceramic coating is either ferrite or chromite, while the preferred valve metal substrates are titanium and niobium.
U.S. Pat. Nos. 3,846,273 and 3,948,751 describe titanium or niobium metal substrates coated with niobium-doped titanium oxide by conventional techniques. U.S. Pat. Nos. 4,112,140 and 4,214,971 disclose a ruthenium oxidetitanium oxide coating on a valve metal substrate produced by conventional techniques.
Later it was discovered that composite anodes having excellent characteristics of low resistivity, very low dissolution rates, long life, durability, and corrosion resistance can be produced by reactive ion plating a thin layer of mixed metal oxides on a selfpassivating electrically conductive valve metal base. The mixed metal oxides are composed of transition metal oxides and/or noble metal oxides of the platinum group. The valve metal is generally titanium or niobium. Three preferred embodiments are a niobium-doped titanium oxide coating, a ruthenium oxide/titanium oxide mixture coating or a iridium oxide/titanium oxide mixture coating on a niobium or titanium valve metal substrate. The thickness of the coating is at least one micron and can be 50 microns or greater. The coating is preferably achieved by reactive ion plating in concert with predeposition sputter cleaning of the substrate surface.
In order to minimize, and preferably eliminate, some of the problems which have caused premature failures of cathodic protection systems, various configurations of ceramic coated impressed current anodes were developed by members of the U.S. Army Construction Engineering Research Laboratory. Paper No. 288, entitled "New Developments in the Ceramic Anode for Cathodic Protection," by Ashok Kumar and Jeffrey Boy, presented at Corrosion 86, Houston, Texas, discloses a new flat titanium plate with a mixed metal oxide coating. The plate was positioned in a polyurethane shield for protection against damaging effects of impacts. While the shield left the coated anode face exposed, it protected the perimeter and the uncoated back of the plate.
Paper No. 71, entitled "New Cathodic Protection Designs Using Ceramic Anodes - For Navigation Lock Gates," by Ashok Kumar and Mark Armstrong, presented at Corrosion 87, San Francisco, California, discloses an improved version of the ceramic coated plate anode. A five inch flat disc, of titanium having a coating of ruthenium oxide/titanium oxide ceramic material on the exposed face, is mounted in the recessed center of a twelve inch fiber reinforced plastic saucer. The space defined by the concave surfaces on the backside of the saucer is filled with polyurethane to provide shock absorbence, as is the space between the backside of the titanium disk and the front side of the saucer. A fiberglass reinforced plastic bolt with a titanium core is used to mount the flat disk anode. The cable to anode connection is made through gold plated titanium pins in a water-tight plug. While this disk electrode does not project outwardly from the hydraulic structure as far as most sacrificial anodes or impressed current button electrodes, it does have a concentrated area of active anode surface. Thus, any damage caused by abrading or impacting contact of ice or debris with the active ceramic anode surface can inactivate a substantial portion of the active anode area.